1. Field of the Invention
The present invention relates to an exhaust emission control catalyst, and particularly, to such a catalyst including an alumina and a catalytic metal carried in the alumina, and a catalyst including: cerium oxide (CeO.sub.2) having a catalyst material as the former catalyst; or a solid acidic aluminosilicate which includes a molecular sieves property and a catalyst material as the former catalyst; and the present invention also relates to a process for producing the exhaust emission control catalyst.
2. Description of the Prior Art
An active alumina having a .gamma.-phase and/or a .eta.-phase has been conventionally used as the above-described alumina, and platinum has been used as the catalytic metal. The aluminosilicate has a function as a support and a hydrocarbon (HC) adsorbing capability (for example, see Japanese Patent Publication No. 27295/1981). The cerium oxide (CeO.sub.2) has a function as a support and a nitrogen oxide (NOx) adsorbing capability. In this case, platinum (Pt) is used as the catalytic metal (for example, see Japanese Patent Application Laid-open No. 184876/1993).
However, the prior art catalyst suffers from a following problem: If platinum (Pt) or palladium is carried in the active alumina, the platinum (Pt) or palladium (Pd) is dispersed, because it is multi-porous and has a large specific surface area. Therefore, the hydrocarbon (HC) adsorbing capability and nitrogen oxide (NOx) adsorbing capability provided by the platinum (Pt) or palladium (Pd) are enhanced, but in an atmosphere containing an excessive amount of air (e.g., an air-fuel ratio A/F.revreaction.24), a complete oxidization of hydrocarbon (HC), namely, a oxidizing reaction represented by HC.fwdarw.CO.sub.2 +H.sub.2 O advances, so that the amount of active aldehyde (CHO) produced, which is a product of partial oxidation of hydrocarbon (HC) and has a NOx reducing capability, is insufficient, and a reduction-impeding effect by oxygen adsorbed on the surface of platinum (Pt) or the like is liable to be produced. For this reason, the reducing conversion of NOx cannot be sufficiently performed, and the range of temperature for conversion of NOx is narrowed.